Mu-opioid receptors are widely distributed in the central and peripheral nervous system and upon activation have manifold actions including; depression of respiration, activation of the reward pathway, disruption of normal gastrointestinal motility and analgesia. Chronic administration of opioids results in a decline in the response (tolerance) the degree of which differs depending on the agonist and the measure under study. Analgesic tolerance to opioids limits the therapeutic efficacy. On the other hand, the development of tolerance in the gut is slow. The link between tolerance measured at the cellular and behavioral levels involves two general mechanisms, one involving a reduction of the receptor/effector coupling and the second results from homeostatic adaptations that counteract opioid signaling. Both mechanisms are initiated by agonist/receptor binding and contribute to behavioral tolerance. One repeatable and robust measure of opioid action in single cells is acute desensitization. This is an initial adaptive step in the pathway to cellular tolerance and is the subject of intense investigation. Most emphasis has centered on a model involving (1) agonist occupancy, (2) receptor phosphorylation by a G- protein Receptor Kinase and (3) arrestin binding. This proposal will study mu- opioid receptors directly using a kinetic approach. Experiments will test the hypothesis that opioid receptor desensitization results from an agonist dependent change in receptor conformation that has high affinity for agonist and is less functional. The experiments will go on to determine how this agonist dependent transition of the receptor is changed following chronic treatment of animals with morphine. The recognition and characterization of the high affinity conformation is a necessary step in developing new approaches to control the development of tolerance to opioids.